It sounds like a paleontological crime scene: dozens of ancient amphibian fossils found buried relatively close together. The bones of the crocodile-sized creatures — known as metoposaurid temnospondyls — lie intact. What brought them there? What killed them? Why did the fossils remain undisturbed?

Researchers report a detailed analysis of the single-largest collection of the species’ fossils found together in Wyoming in the journal PLOS ONE. But the survey of what appears to be a mass die-off provokes more questions than it provides answers.

Mass Die-Off of Ancient Amphibians

The site is especially interesting because it appears to offer what Aaron Kufner, a University of Wisconsin, Madison paleontologist and first author of the study, called in a press release “…a snapshot of a single population rather than an accumulation over time.”

Although the collection more than doubles the known number of individual Buettnererpeton bakeri fossils, it doesn’t provide any solid answers about what brought them together — or what ultimately killed them.

They did determine that the burial site was likely a floodplain, based on ancient grains of sand and finely layered strata of sediment. But the cause of death remains elusive. It appears unlikely that they drowned in a current because some of the most delicate parts of the fossils appear well-preserved. Also, the pattern of the fossils’ placements doesn’t indicate that any amphibians had been swept away.

The amphibians, whose dense shoulder bones made them especially adept at sinking, likely submerged themselves in freshwater bodies and waited for unsuspecting prey. The researchers aren’t sure if the amphibians clustered together to hunt as a group or to breed. They theorize that it is possible that they somehow become entrapped in the area, perhaps because of a blocked waterway. But they don’t know for sure.

Read More: Dense Bones Helped This 225-Million-Year-Old Ancient Amphibian To Sink

Other Fossil Beds

Researchers also don’t know if this mass die-off was a singular event or a fate only this particular group suffered. Other fossil beds contain multiple metoposaurids but largely reveal a more scattered collection of bones that were likely moved by wind, water, or other forces.

“Whether or not this kind of die-off was common among temnospondyls is difficult to assess since few other sites have received this level of detailed examination and there is limited data regarding how temnospondyl burial varies with different environments,” according to the press release.

Read More: This Tomb Contains 10 Mummified Crocodiles From 2,500 Years Ago

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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.